1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more specifically relates to an image forming apparatus for forming images with reversal development.
2. Description of the Related Art
Image forming methods using electrophotographic processes have generally found wide use in conventional copying machines, and are also used in printers that use a laser beam to write digital data on a photosensitive member.
Digital printers that form electrophotographic images must use a reversal developing method for character writing wherein the parts of the photosensitive member relative to recording data are exposed by a laser beam. Reversal developing methods develop electrostatic latent images formed on the surface of a photosensitive member charged with a specific polarity by means of a toner having the same polarity when a developing bias voltage VB is applied.
FIG. 1 shows a conventional image forming device using a reversal development method. In the aforesaid conventional device, when the main motor (not shown in the drawing) is switched ON, the cylindrical photosensitive member 1 rotates and the main charger 2, which is a scorotron type charger with an attached grid 2b, is simultaneously switched ON so as to charge the surface of photosensitive member 1 with an initial surface electrical potential V.sub.o, as shown in the timing chart in FIG. 2.
Thereafter, the charged surface of the photosensitive member I is irradiated by a laser beam or like image light L by means of the exposure means 6 so as to form an electrostatic latent image on the surface of photosensitive member 1. Then, the aforesaid electrostatic latent image is developed into a toner image on the surface of photosensitive member 1 by the developing roller 3 using a reversal development method.
On the other hand, a recording sheet (not shown in the drawing) is taken up from a paper supply device (also not shown in the drawing) and guided to a pair of timing rollers 4, and transported to the transfer charger 5 so as to arrive simultaneously with the toner image with a timing that is coordinated by switching ON the timing rollers 4.
The transfer charger 5 is switched ON with a timing that coincides with the arrival of the recording sheet, and transfers the toner image to the recording sheet by means of an electrical charge having a polarity which is opposite to the polarity of the main charger 2. The operation of the aforesaid devices is controlled by a central processing unit (CPU).
FIGS. 4 and 5 shows the measured values of surface potentials V.sub.i of the image region on the surface of the photosensitive member 1 after exposure to light when each single cycle of image formation with reversal development is repeated. That is, at the beginning of image formation when a relatively few number of images are formed, the value of electric potential V.sub.i of the image portion becomes V.sub.ia, as shown in FIG. 4 and the difference .DELTA.V between the developing bias and the electric potential V.sub.ia is relatively small. However, the electric potential V.sub.i of the image portion decreases as the number of formed images increases. When a few hundreds of image formation are accomplished, the electric potential of the image region becomes V.sub.ib, and the difference in the electric potential .DELTA.V becomes larger. Since the sensitivity of photosensitive member 1 becomes greater as the difference in electric potential .DELTA.V becomes larger, the photosensitivity of photosensitive member 1 increases as the number of images formed increases, in a process of so-called memory-sensitized high-speed developing.
Particularly when an organic photosensitive member is used, the post-exposure potential V.sub.i of the image portion of the surface of the photosensitive member 1 is greatly reduced as the number of images formed increases, as shown with a broken line in FIG. 6. Therefore, the density of the formed image gradually increases and the image line width becomes gradually thicker, such that uniform images cannot be produced.
As shown in the timing chart in FIG. 3, the main motor and main charger 2 are switched ON so that as the photosensitive member 1 is rotated said member is simultaneously and continuously charged by main charger 2, and, similarly, the exposure means timing rollers 4, transfer charger 5 and the like are sequentially switched ON at set intervals so as to repeatedly form images and develop said images with a reversal development process. In this case, just as in the previously described single-cycle image formation process, the post-exposure potential V.sub.i on the surface of photosensitive member 1 is markedly reduced as the number of formed images increases (refer to FIG. 7), and the previously mentioned disadvantages also occur.